The present invention generally relates to tools used in subterranean wells and, in a preferred embodiment thereof, more particularly relates to apparatus and methods for conducting perforation and related formation fracturing operations in subterranean wells.
A potentially productive geological formation beneath the earth's surface often contains a sufficient volume of valuable fluids, such as hydrocarbons, but also has a very low permeability. "Permeability" is a term used to describe that quality of a geological formation which enables fluids to move about in the formation. All potentially productive subterranean formations have pores, a quality described using the term "porosity", within which the valuable fluids are contained. If, however, the pores are not interconnected, the fluids cannot move about and, thus, cannot be brought to the earth's surface without a structural modification of the production zone.
When such a formation having very low permeability, but a sufficient quantity of valuable fluids in its pores, is desired to be produced, it becomes necessary to artificially increase the formation's permeability. This is typically accomplished by "fracturing" the formation, a practice which is well known in the art and for which purpose many methods have been conceived. Basically, fracturing is achieved by applying sufficient pressure to the formation to cause it to crack or fracture, hence the term "fracturing" or simply "fracing". The desired result of this process is that the cracks interconnect the formation's pores and allow the valuable fluids to be brought out of the formation and to the surface.
Using previously proposed apparatus and methods, the general sequence of steps needed to stimulate a production zone through which a wellbore extends is as follows. First, a perforable nipple is made up in the well casing, and cemented in, at a predetermined depth in the well--i.e., within the subterranean production zone requiring stimulation. Next a perforating trip is made by lowering a perforation assembly into the nipple on a lower end portion of a tubular workstring. The gun assembly is they detonated to create a spaced series of perforations extending outwardly through the nipple, the cement and into the production zone. The discharged gun assembly is then pulled up with the workstring to complete the perforating trip.
Next, the spent gun assembly is replaced on the workstring with a tubular proppant discharge member having a spaced series of sidewall proppant slurry discharge openings formed therein, the discharge openings being at least theoretically alignable with the gun-created perforations extending outwardly through the now perforated nipple in the well. With the proppant discharge member in place, the workstring is again lowered into the well (typically with one or more stimulation packers thereon) until the proppant discharge member is within the nipple. Proppant slurry is then pumped down the workstring so that proppant slurry is discharged through the discharge member side wall outlet openings and then flowed outwardly through the nipple and cement perforations into the corresponding perforations in the surrounding production zone. The workstring is then pulled out again to complete the stimulation trip and ready the casing for the installation therein of production tubing and its associated production packer structures.
This previously proposed perforation and proppant fracturing technique has several well known and heretofore unavoidable problems, limitations and disadvantages. For example, it requires two separate trips into the well to respectively carry out the necessary perforation and fracturing procedures.
Additionally, when the proppant slurry discharge member is lowered into the perforated nipple it is, as a practical matter, substantially impossible to obtain a precise alignment (in both axial and circumferential directions) between the side wall discharge openings in the proppant slurry discharge member and the gun-created perforations in the nipple. The usual result of this discharge opening/nipple perforation misalignment is that after it is discharged from the workstring, the proppant must follow a tortuous path on its way to entering the nipple perforations. Because of the highly abrasive character of proppant slurry, this tortuous flow path can easily cause severe abrasion wear problems in the casing.
Using this previously proposed perforation and proppant fracturing technique also limits the ability to isolate multiple production zones from one another--a requirement that may easily arise due to the fact that different zones may require different fracturing pressures and total amounts of proppant. This problem can be partially alleviated by using straddle packers at each zone. However, each zone requires a separate trip with packers, and the retrieval of the packers can be quite difficult.
Moreover, there is a lack of immediate (i.e., right after proppant fracturing) proppant flow-back control. After the production zone is stimulated using this technique, proppant flow-back can easily occur when the proppant pumping pressure is relaxed, or later when the well is producing. Such proppant flow-back creates a variety of problems, such as abrasion of production equipment, or reduction in the production rate of the stimulated formation.
Finally, the previously proposed perforation and proppant fracturing technique described above lacks the ability to provide well pressure balance control during pre-production trips, thereby tending to create undesirable unbalanced pressure situations during the completion of the well.
As can be readily seen from the foregoing, it would be highly desirable to provide improved perforation and proppant fracturing apparatus and methods which eliminate or at least substantially reduce the above-mentioned problems, limitations and disadvantages commonly associated with the previously proposed perforation/stimulation technique generally described above. It is accordingly an object of the present invention to provide such improved apparatus and methods.